An ultrasonic measuring apparatus includes a transmitter to send out an ultrasonic wave and a receiver to receive the ultrasonic wave and estimates the distance between the transmitter and the receiver by the amount of time it has passed since the transmitter sent out the ultrasonic wave and until the receiver receives the ultrasonic wave. Alternatively, another ultrasonic measuring apparatus may estimate the distance between an object and the ultrasonic measuring apparatus itself by the amount of time it has taken for an ultrasonic wave, sent out from the transmitter, to reach the object, get reflected by the object and then get received at the receiver.
In an environment where there are a number of ultrasonic measuring apparatuses, the ultrasonic signals transmitted by the respective ultrasonic measuring apparatuses would interfere with each other, thus possibly causing measurement errors if no countermeasures were taken. To avoid such a situation, somebody proposed a method for preventing the ultrasonic signals from interfering with each other by making the ultrasonic measuring apparatuses transmit the ultrasonic signals at mutually different times. Another person proposed a method for distinguishing the ultrasonic signals from each other by encoding the ultrasonic signals, generated by the respective ultrasonic measuring apparatuses, with mutually different codes.
The former method is called “time-division transmission” and has been used as a simple method. However, if the respective ultrasonic measuring apparatuses were independent of each other, it is impossible for any of the apparatuses to know when the other ultrasonic measuring apparatuses will transmit the ultrasonic signals. That is why it is difficult to adjust the timings to transmit the ultrasonic signals so as to not to cause any interference.
A conventional ultrasonic measuring apparatus that adopts the latter method is disclosed in Patent Document No. 1, for example. FIG. 22 is a block diagram showing the conventional ultrasonic measuring apparatus disclosed in Patent Document No. 1. Hereinafter, the basic operation of this conventional ultrasonic measuring apparatus 101 will be described. The ultrasonic measuring apparatus 101 includes a transmitter 8, a receiver 9, a correlator 103, a peak detector 104 and a pulse generator 105.
The pulse generator 105 generates a drive signal for the transmitter 8, which sends out an ultrasonic signal into the space. The ultrasonic signal transmitted passes through an ultrasonic wave propagation path 7 to reach an object 2 and get reflected by the object 2. The ultrasonic signal reflected passes through the ultrasonic wave propagation path 7 again to reach the receiver 9.
The drive signals are encoded with mutually different codes by respective ultrasonic measuring apparatuses so as to be identified from each other even when the ultrasonic signals sent out by those apparatuses interfere with each other. Considering such a situation where a desired signal should be decrypted and extracted from a number of signals interfering with each other, the other signals are preferably quite dissimilar from the desired one. Random signals that are generated artificially under a predetermined rule so as to have such a characteristic are called “pseudo random signals”.
Digital signals represented as a combination of zeros and ones are often used as the pseudo random signals because digital signals are easy to process. Examples of known digital pseudo random signals include an M-sequence, a Barker sequence and a Golay sequence. Among other things, the M-sequence functions as a code for use in a telecommunications system that adopts the spread spectrum technology, i.e., a noise-like identifiable carrier for the information to be transmitted. One of two different M-sequences looks nothing but noise for the other. That is why it is very effective to extract its own signal. Also, even if there are two identical M-sequences, one of the two also looks nothing but noise for the other when there is even a slight time lag between them. As a result, it can be seen at what time a particular M-sequence is present among the time series of those interfering received signals.
The drive signal generated by the pulse generator 105 is a spread spectrum (M-sequence discrete) random wave. Patent Document No. 1 realizes a pseudo random signal with such a characteristic by binary frequency shift keying in which the frequency associated with bit one and the frequency associated with bit zero are different from each other.
The ultrasonic wave that has left the transmitter 8, passed through the ultrasonic wave propagation path 7 and then reached the receiver 9 has its correlation with the pseudo random signal, generated by the pulse generator 105, examined by the correlator 103. The peak detector 104 detects the peak of the correlation value. The time when the correlation value reaches its peak represents the time when the ultrasonic wave, sent out from the transmitter 8, reaches the receiver 9. And the interval between the time when the ultrasonic wave was transmitted and the time when the correlation value reaches its peak represents the propagation time of the ultrasonic wave to the object 2. Consequently, the distance from the ultrasonic measuring apparatus 101 to the object 2 can be measured by the propagation velocity of the ultrasonic wave.
The (M-sequence discrete) spread spectrum pseudo random signals are unique signals for respective ultrasonic measuring apparatuses. That is why even if an ultrasonic wave that has been sent out from another ultrasonic measuring apparatus reaches the receiver 9, its correlation with the pseudo random signal generated by the pulse generator 105 is very little. Consequently, no peak is detected by the correlator 103 and the ultrasonic measuring apparatus 101 can identify a pseudo random signal that has come from another ultrasonic measuring apparatus.
In the field of radio communications, on the other hand, to maintain good communications even in an environment in which a lot of signals are interfering with each other, the code of a signal to transmit is made redundant and used for error correction or the interference is canceled by subtracting extra signals (i.e., signals other than its own signal) from the received signal according to known techniques. Particularly, in the field of cellphones, to let the base station cancel the interference between the signals that have been encoded with mutually different codes by individual users, various interference cancellers have been proposed. For example, Patent Document No. 2 discloses an interference canceller for use in the base station of cellphones.
FIG. 23 is a block diagram showing the interference canceller disclosed in Patent Document No. 2, which includes an interference canceller block 106 and a demodulation processing section 107. The interference canceller block 106 includes delay processing sections 108(1) through 108(N) for causing delays in the received signal and interference canceling sections 109(1) through 109(N) for extracting extra signals, other than the desired one, from the received signal.
In the interference canceller block 106, the interference canceling sections 109(1) through 109(N) makes replicas of the other extra signals, except the desired one, using all N types of codes but the one used for the desired signal, and subtracts the replica signals, other than the desired one, from the received input signal 110. In this case, the delay processing sections 108(1) through 108(N) delays the received signal 110 until the outputs of the interference canceling sections 109(1) through 109(N) are obtained and such that the outputs of the interference canceling sections 109(1) through 109(N) are synchronized with the received signal 110 during the subtraction process.
The demodulation processing section 107 performs demodulation processing on the desired signal using this output signal. As a result, the interference caused by the extra signals, other than the desired one, can be canceled and the signal quality can be improved compared to a situation where demodulation is performed without using the interference canceller block 106. If this series of processing is carried out repeatedly a number of times by changing this interference canceller block 106 into a multi-stage one, then the interference can be canceled even more effectively.
The information transmitted from a cellphone has been encoded by being multiplied by a pseudo random signal such as an M-sequence signal. This processing will be referred to herein as “spreading processing”. The encoded information is transmitted on a sine wave called a “carrier”. In that case, the phases (e.g., 0 degrees and 180 degrees) of the sine wave correspond to one and zero of the encoded information.
In the base station, to retrieve information from the signal that has been transmitted from the cellphone, despreading processing, which is the inverse processing of the spreading processing, is carried out. It is determined by reference to the phase information of a despread signal whether the signal is zero or one. This processing will sometimes be referred to herein as “demodulation”.
FIG. 24 is a block diagram showing how the interference canceling section 109(N) operates. The interference canceling section 109(N) includes a number K (where K is an integer and equal to or greater than one) of despreading blocks 111(1) through 111(K) and spreading blocks 117(1) through 117(K) for multiple propagation paths and a decision section 116. The information about the multiple propagation paths should be collected in advance by predicting what multiple paths there will be. The despreading block 111(1) includes a despreading section 112(1), a complex conjugating section 113(1), a propagation path estimating section 114, and a multiplier 115(1). On the other hand, the spreading block 117(1) includes a multiplier 119(1) and a spreading section 118(1). Each of the other despreading blocks 111(2) through 111(K) also includes the same components as the despreading block 111(1) and each of the other spreading blocks 117(2) through 117(K) also includes the same components as the spreading block 117(1).
The despreading section 112(K) despreads the received signal using the spread code of the nth user (where n is an integer and 1≦n≦N) synchronously with the kth one (where k is an integer and 1≦k≦K) of the multiple propagation paths. The output of the despreading section 112(k) is supplied to the multiplier 115(k), which receives the propagation path characteristic from the propagation path estimating section 114(k) by way of the complex conjugating section 113(k) and multiplies the output of the despreading section 112(k) and the propagation path characteristic together. The propagation path estimating section 114 outputs known information called a “pilot signal” (such as the repetition of 101010) before providing the propagation path characteristic. Thus, it can be estimated how much the magnitude of the signal has varied and how long the signal has been delayed by going through the propagation path after the despreading. The estimated values are complex numbers. The time lags on all of the multiple propagation paths are adjusted using these values.
At the same time, the multiplier 115 weights the amplitude of the output of the despreading section 112 with the output of the complex conjugating section 113(k) for the purpose of rake combination (i.e., combining maximum ratios with each other). The radio waves that have propagated through the respective paths arrive at mutually different times due to the difference in their path length. The technique of increasing the responsively by eliminating such time lags and adding together all radio waves that have propagated through the respective paths is called “rake combination”.
Thereafter, the weighted detected outputs supplied from the respective multipliers 115 for the K paths are subjected to the rake combination and then the combined output is supplied to the decision section 116, which determines the most likely transmission symbol (i.e., whether it is zero or one).
The spreading blocks 117 are connected in series to the K paths, respectively. The output of the decision section 116 and the propagation path characteristic, which is the output of the propagation path estimating section 114, are multiplied together by the multiplier 119(K) on a path-by-path basis. The spreading section 118 spreads the output of the multiplier 119(K) using the spread code of the nth user synchronously with its associated path (i.e., the kth path) of the multiplier. Then, the replica signals for the respective paths, which are the K outputs of the spreading sections 118, are combined with each other, thereby obtaining a replica signal for the nth user.
The interference canceller ranks the reception levels of all users in advance and performs the demodulation and interference canceling processing at each stage in the descending order of the levels. The reception levels may be ranked either only once using the received signal or a number of times sequentially at the respective stages using the interference canceling signal.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-108826        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 10-51353        